Stars in star clusters move on very similar trajectories through the Milky Way since the internal motion of stars in the cluster is usually two orders of magnitudes smaller than the velocity with which the
cluster moves around the Galactic centre. As the distance and viewing angle to the stars changes along the orbit, the proper motion and line-of-sight velocity of the stars will change as well and one can
use this variation to measure the distance to the cluster. This is the so-called moving group or convergent point method [1]}, [2]}. To first order, the change in proper motion \(\mu \) 
due to these perspective effects can be approximated by
\(\mu = -\frac{v_{Los} \alpha }{D A} \;\; \mbox{mas yr}^{-1}\) 
